U.S. patent number 8,821,035 [Application Number 14/018,951] was granted by the patent office on 2014-09-02 for optical element and system using the same.
This patent grant is currently assigned to Flir Systems, Inc.. The grantee listed for this patent is FLIR Systems Trading Belgium BVBA. Invention is credited to Eric G. Johnson, Alan D. Kathman, Charles S. Koehler, Robert D. TeKolste, William H. Welch.
United States Patent |
8,821,035 |
Kathman , et al. |
September 2, 2014 |
Optical element and system using the same
Abstract
A method for transmitting a signal in an optical system includes
generating an optical signal along an optical axis for transmission
through an optical element, positioning the optical element so that
a surface discontinuity is positioned along the optical axis such
that the optical signal defines a substantially radially symmetric
intensity profile, and launching the optical signal into an input
face of an optical fiber such that the intensity profile is
substantially null proximate an optical axis associated with the
optical fiber.
Inventors: |
Kathman; Alan D. (Charlotte,
NC), Koehler; Charles S. (Concord, NC), Welch; William
H. (Charlotte, NC), Johnson; Eric G. (Harrisburg,
NC), TeKolste; Robert D. (Charlotte, NC) |
Applicant: |
Name |
City |
State |
Country |
Type |
FLIR Systems Trading Belgium BVBA |
Meer |
N/A |
BE |
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Assignee: |
Flir Systems, Inc.
(Wilsonville, OR)
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Family
ID: |
45870755 |
Appl.
No.: |
14/018,951 |
Filed: |
September 5, 2013 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20140112619 A1 |
Apr 24, 2014 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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12805478 |
Aug 2, 2010 |
8529139 |
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12453229 |
May 4, 2009 |
7769258 |
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12073748 |
Mar 10, 2008 |
7529446 |
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11802044 |
May 18, 2007 |
7343069 |
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10320525 |
Dec 17, 2002 |
7221823 |
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09614184 |
Jul 11, 2000 |
6496621 |
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09329996 |
Jun 11, 1999 |
6530697 |
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60101367 |
Sep 22, 1998 |
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Current U.S.
Class: |
385/88; 385/39;
385/14; 385/33 |
Current CPC
Class: |
G02B
6/26 (20130101); G02B 6/4204 (20130101); G02B
6/4206 (20130101); G02B 6/34 (20130101); G02B
6/4203 (20130101); G02B 6/4214 (20130101) |
Current International
Class: |
G02B
6/36 (20060101) |
Field of
Search: |
;385/33,36,88-89 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0 595 449 |
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May 1991 |
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EP |
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2 252 843 |
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Aug 1992 |
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GB |
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4-034505 |
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Feb 1992 |
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JP |
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WO 00/13051 |
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Mar 2000 |
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WO |
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Primary Examiner: Wong; Eric
Attorney, Agent or Firm: Lee & Morse, P.C.
Parent Case Text
CROSS-REFERENCES TO RELATED APPLICATIONS
The present application is a division of pending application Ser.
No. 12/805,478, filed Aug. 2, 2010, which in turn is a
continuation-in-part of application Ser. No. 12/453,229, filed May
4, 2009, now U.S. Pat. No. 7,769,258 on Aug. 3, 2010, which is a
continuation of application Ser. No. 12/073,748, filed Mar. 10,
2008, now U.S. Pat. No. 7,529,446, which in turn is a continuation
of Ser. No. 11/802,044, filed May 18, 2007, now U.S. Pat. No.
7,343,069 B2, which in turn is a continuation of Ser. No.
10/320,525, filed Dec. 17, 2002, now U.S. Pat. No. 7,221,823 B2,
which is a continuation of Ser. No. 09/614,184, filed Jul. 11,
2000, now U.S. Pat. No. 6,496,621, which is a continuation-in-part
of U.S. patent application Ser. No. 09/329,996, filed Jun. 11,
1999, now U.S. Pat. No. 6,530,697, which claims priority under 35
U.S.C..sctn.119 to Provisional Application No. 60/101,367 filed on
Sep. 22, 1998, the entire contents of all of which are hereby
incorporated by reference for all purposes.
Claims
What is claimed is:
1. An optical system comprising: an optical element having an input
surface, an output surface, and an optical axis; and an optical
medium having an end proximate to the output surface of the optical
element and an end distal to the output surface of the optical
element, the optical medium configured to receive emitted light
having a received annular intensity profile and transfer the light
to the distal end such that the transferred light substantially
retains the received annular intensity profile at the distal end;
and wherein: the output surface has a continuous slope/curvature in
an area approximately between a periphery of the output surface and
a first distance from the optical axis; and the output surface has
a discontinuity in an area within a perimeter of the first distance
for emitting light having an output annular intensity profile
receivable at the proximate end of the optical medium.
2. The system of claim 1, wherein the optical element is a mode
matching element, wherein light output from the optical element is
distributed in a desired angular distribution which is
substantially maintained along the fiber for more than a depth of
focus of the optical element.
3. The system of claim 2, wherein said mode matching element is a
diffractive element.
4. The system of claim 2, wherein said mode matching element is a
refractive element.
5. The system of claim 2, wherein said optical element comprises
first and second surfaces, said mode matching element being
provided on said second surface, further from the light source.
6. The system of claim 5, further comprising an angular
distribution altering element on said first surface.
7. The system of claim 6, wherein said angular distribution
altering element provides a ring pattern on said second
surface.
8. The system of claim 6, wherein the angular distribution altering
element is a diffractive surface having a radially symmetric lens
function and a negative axicon function.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
Embodiments of the present invention are directed to an optical
element having both an amplitude diffractive structure and a phase
diffractive structure and/or a vortex lens.
2. Description of Related Art
It is very difficult to manufacture a multi-mode fiber with good
control over the index of refraction in the center of the fiber. If
the light coupled to the fiber excites some modes that propagate
mostly in the center of the fiber and other modes which do not
propagate mostly in the center of the fiber, very different
propagation times for these modes may result. This is referred to
as differential mode delay. Differential mode delay tends to spread
out the pulse length of signals and reduce the effective bandwidth
of the fiber.
Modes which propagate mostly in the center of the fiber are the
lower order fiber modes, i.e., modes having small propagation
angles that strike at or near the center of the fiber. These lower
order modes spend most of the time in the center of the fiber, tend
to travel straight down the fiber and the shape of these modes does
not change much as they propagate. Therefore, in order to reduce
differential mode delay, any light which enters near the center of
the fiber needs to be incident at an angle which is large enough
not to excite lower order modes, but not so large that the critical
angle is exceeded and the light fails to be coupled or no light
should be input to the center of the fiber.
One current solution involves coupling light into single mode
fibers which are then positioned off-axis relative to the
multi-mode fiber. Single mode fibers have a much smaller core than
multi-mode fibers, so can be used to provide light at specific
positions on the endface of the multi-mode fiber. However, single
mode coupling is more expensive than multi-mode coupling and the
additional coupling step leads to an increase loss in light.
Further, while no light enters the fiber of the center for this
configuration, the light will still cross the fiber axis as it
propagates, thus increasing the differential mode delay.
Additionally, ferrules or other structures housing the multi-mode
fiber to a single mode fiber junction are not readily available and
must be developed specifically for that purpose.
Another solution is to use a vertical cavity surface emitting laser
(VCSEL) excited to radiate in a ring mode. The operation of the
VCSEL in radiation modes other than the lowest order have less
predictable flux distributions than in the lowest order mode, in
which the distribution more closely approximates a Gaussian
profile. Further, there will still be some power in the lower order
modes of the VCSEL. Additionally, such operation of the VCSEL often
requires a higher current to drive the source into the higher
radiation modes.
As the use of non-physical contact connections between light
sources and fibers increases, the need for effective isolation to
prevent light reflected at the fiber interface from being returned
to the light source increases. Feedback to the light source may
result in spectral broadening, light source instability, and
relative intensity noise, which affect the monochromaticity of the
light source. As data rates go up, the systems become more
sensitive to relative intensity noise and require low bit error
rates. Conventional optical isolators using polarization effects to
attenuate reflection are very expensive, making the non-physical
contact impractical. The importance of avoiding feedback is further
increased when trying to use cheaper light sources, such as
vertical cavity surfaces emitting laser diodes and light emitting
diodes.
One solution that avoids the use of an optical isolator is a mode
scrambler that divides power from the light source into many modes.
A configuration employing a mode scrambler includes a single mode
pigtail that provides light from the light source to the mode
scrambler that then delivers the light to a transmission cable via
an air-gap connector. Since any reflected power will still be
divided across the many modes, any reflected power in the mode that
can efficiently be coupled into the pigtail is only a small
fraction of the total reflected power, thereby reducing return
losses. However, this solution involves aligning another fiber,
physically contacting the fiber with the mode scrambler, and
placing the light source against the fiber. This pigtailing is
expensive. Thus, there still exists a need for true non-physical
contact connection between a light source and a transmission system
that does not require an isolator.
SUMMARY
Embodiments are therefore directed to an optical element that
substantially overcomes one or more of the problems due to the
limitations and disadvantages of the related art.
Embodiments may be directed to providing a method for transmitting
a signal in an optical system, the method including generating an
optical signal along an optical axis for transmission through an
optical element, positioning the optical element so that a surface
discontinuity is positioned along the optical axis such that the
optical signal defines a substantially radially symmetric intensity
profile, and launching the optical signal into an input face of an
optical fiber such that the intensity profile is substantially null
proximate an optical axis associated with the optical fiber.
An axicon function may be provided with respect to the optical
signal via the surface discontinuity.
The relative minimum of the incident field amplitude of the
intensity profile may be located proximate a center of the
intensity profile.
The intensity profile may be substantially symmetric about the
relative minimum of the incident field amplitude.
The intensity profile may be substantially annular.
Embodiments may be directed to providing an optical system
including an optical element having an input surface, an output
surface, and an optical axis, and an optical medium having an end
proximate to the output surface of the optical element and an end
distal to the output surface of the optical element, the optical
medium configured to receive emitted light having a received
annular intensity profile and transfer the light to the distal end
such that the transferred light substantially retains the received
annular intensity profile at the distal end. The output surface may
have a continuous slope/curvature in an area approximately between
a periphery of the output surface and a first distance from the
optical axis. The output surface may have a discontinuity in an
area within a perimeter of the first distance for emitting light
having an output annular intensity profile receivable at the
proximate end of the optical medium.
The optical element may be a mode matching element, wherein light
output from the optical element is distributed in a desired angular
distribution which is substantially maintained along the fiber for
more than a depth of focus of the optical element.
The mode matching element may be a diffractive element or a
refractive element.
The optical element may include first and second surfaces, the mode
matching element being provided on said second surface, further
from the light source. An angular distribution altering element may
be on the first surface. The angular distribution altering element
may provide a ring pattern on said second surface. The angular
distribution altering element may be a diffractive surface having a
radially symmetric lens function and a negative axicon
function.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other features and advantages will become more
apparent to those of ordinary skill in the art by describing in
detail exemplary embodiments with reference to the attached
drawings, in which:
FIG. 1 illustrates the integration of the coupler of an embodiment
with a light source, a fiber and a light source power monitor;
FIGS. 2A-2C illustrate a diffractive element and associated
characteristics of a spiral generator for use as the coupler in
accordance with an embodiment;
FIG. 3 is a schematic illustration of another embodiment of the
coupler;
FIG. 4 is a schematic illustration of another embodiment of the
coupler;
FIG. 5A illustrates a schematic view according to an
embodiment;
FIG. 5B illustrates a schematic view according to an
embodiment;
FIG. 6 illustrates a radiation profile of light traversing an
embodiment of FIG. 5A or 5B;
FIG. 7 illustrates another embodiment of the optical element;
and
FIGS. 8A, 8B and 9 illustrate other embodiments of the optical
element and/or beam shaper.
DETAILED DESCRIPTION
FIG. 1 illustrates a light source 10, here a VCSEL, a coupler 12
and a multi-mode fiber 14 integrated with a power monitor 16 and a
reflective surface 18 for directing the light into the fiber 14. In
particular, the light source 10 and the power monitor 16 are
provided on a substrate 20. Another substrate 22 has the coupler 12
thereon, preferably on the face furthest from the light source to
allow the beam to expand, and a splitting diffractive element 24
which splits off a portion of the light from the light source 10 to
be monitored. The substrates 20, 22 are preferably mounted with
spacer blocks 26, which provide the desired separation between the
substrates 20, 22. The coupler 12 may also be provided in a common
housing with the fiber 14.
The light split off by the diffractive element 24 is directed to
the power monitor 16 to monitor the operation of the light source
10. The directed of the light to the power monitor 16 may be
achieved by providing appropriately positioned reflective portions
28. The number of times the light to be monitored traverses the
substrate 22 is a design choice, depending on the initial angle of
diffraction and the desired positioning of the power monitor 16.
This monitoring is set forth in commonly assigned U.S. application
Ser. No. 09/386,280, entitled "A Diffractive Vertical Cavity
Surface Emitting Laser Power Monitor and System" filed Aug. 31,
1999, which is hereby incorporated by reference in its entirety for
all purposes. Alternatively, the power monitoring may be realized
using an integrated detector, without the need for the deflecting
element, as set forth in commonly assigned U.S. application Ser.
No. 09/548,018, entitled "Transmission Detection for Vertical
Cavity Surface Emitting Laser Power Monitor and System" filed Apr.
12, 2000, which is hereby incorporated by reference in its entirety
for all purposes
The light that is not split off by the diffractive element 24
proceeds to the coupler 12. A reflective surface 18, such as a
polished angular face of another substrate, is provided to direct
the light from the coupler 12 into the multi-mode fiber 14.
Preferably all the optical elements are formed lithographically and
all the elements are integrated on a wafer level.
In accordance with the present invention, the coupler 12 is a
diffractive element that matches the phase as well as the intensity
distribution of the beam. The matching of the phases generates
spiral propagation of the beam through the fiber. This spiral or
vortex propagation maintains the intensity profile input to the
fiber along the fiber. Since the beam travels in a corkscrew, the
amount of light crossing the center of the fiber is significantly
reduced. Ideally, the amount of light in the center will be zero,
but in practice, the amount of light is on the order of 10% or
less. In contrast, when only the intensity distribution is
controlled, as in the first two designs of the parent application,
the input intensity profile may be the desired profile, but will
quickly degrade as the light traverses the fiber, In other words,
while the other designs may provide an input profile that is
substantially null on axis, this profile is only maintained for the
depth of focus of the coupler. When also matching the phase, this
profile is maintained substantially beyond the depth of focus of a
lens having the same numerical aperture as the beam to be input to
the fiber, e.g., at least an order of magnitude longer. Absent the
fiber, the null space of the beam profile is maintained through
free space, which significantly reduces the alignment requirement.
Further, by matching the phase and amplitude of the beam to a
certain mode of the fiber, theoretically the beam profile could be
maintained over an infinite length of fiber. However, imperfections
in the real world, e.g., in the fiber, in the beam, in the
matching, degrade from this theoretical scenario.
Thus, in order to avoid low order modes in a GRIN fiber launch, the
amplitude and phase of the higher order modes need to be matched.
The following equations are set forth in Fields and Waves in
Communication Electronics, Simon Ram et al. 1984, particularly pp.
765-768, which is hereby incorporated by reference in its entirety.
For a GRIN fiber, these eigenmodes all have the form set forth in
Equation (1):
E(r,.theta.,z).varies.f.sub.mp(r)e.sup..+-.jm.theta.e.sup..+-.j.beta..sup-
.mp.sup.z (1)
where f(r) is a function that depends only on r for given modes
within a specific fiber, r is the radius from the axis, .theta. is
the angle from the axis, z is the distance along the axis, m is the
azimuthal mode number, .beta. is a propagation constant, p is the
radial mode number. When m, p=0, the beam has a Gaussian
profile.
While Equation (1) could be used to match a particular mode of the
fiber by creating an input light beam having an amplitude and phase
function which exactly correspond to the particular mode, such
matching is not required and may not even be desirable, as matching
the amplitude as well as the phase increases the requirements on
the optics. As long as m>0, the azimuthal mode m will have a
phase function that is a spiral ring, whether the light is
traveling in free space or in a fiber. Once the phase function for
at least one higher order mode, i.e., m>0, has been matched, a
null at the center of the beam is created after the beam having
been phase matched propagates over a short distance, e.g., a few
wavelengths. Unlike other types of matching, this null is
maintained in the center in both free space and the fiber, so such
an optical element providing such matching does not have to be
immediately adjacent to the fiber. As evident from Equation (1),
when matching the phase, the value of p doesn't matter.
In order to suppress the lowest order mode, i.e., m=0, a phase term
needs to be added to the wavefront. This is accomplished through
the use of the following diffractive phase function encoded onto
the wavefront set forth in Equation (2):
.PHI..function..times..times..function. ##EQU00001##
where .phi. is the phase function, x and y are the coordinates in
the plane. In general, there will be several modes propagating,
e.g., m=1-5. The spiral mode may be realized by matching the phase
function for m=3.
This phase function can be added to a lens function and encoded as
a mod(2.pi.) diffractive element as set forth in Equation (3):
.PHI..function..pi..function..lamda..times..times..times..times..function-
. ##EQU00002##
FIG. 2A illustrates the mod(2.pi.) diffractive element and the
corresponding intensity to in the focal plane of the lens function.
FIG. 2B illustrates an actual example of a diffractive optical
element 12 created in accordance with Equation (3). FIG. 2C
illustrates the simulated ring intensity 25 and the measured
intensity pattern 29 of the element 12 in FIG. 2B. A refractive
equivalent in accordance with Equation (3) of the phase matching
diffractive 12 may be alternately employed.
This phase matching coupler 12 is not a true beam shaper, since
each point in the input plane is mapped into more than one point in
the output plane because of the axial singularity. Unlike a
diffuser, each point in the input plane is not mapped to every
point in the output plane.
The phase matching coupler 12 allows the desired angular
distribution to be substantially maintained along a portion of the
fiber. This may be quantified by measuring the amount of power
within a certain radius of the fiber at a certain distance along
the fiber. The phase matching of the present invention allows more
power to be contained within the desired radii for a longer
distance than methods not employing phase matching. For example, by
aligning the coupler and a GRIN fiber along the same axis, using a
850 nm source, and matching both the phase and the amplitude, the
encircled energy can be maintained to less than 12.5% is a radius
of less than 4.5 microns and 75% for a radius less than 15 microns,
with no power in the fiber center, for over 6 m.
By matching the phases, the light from the coupler is input to the
fiber traveling in a circular direction, i.e., the path of the
light down the fiber forms a corkscrew. Such traversal is opposed
to the linear travel normally occurring down the fiber. By
traveling in a corkscrew or spiral mode, the input distribution,
typically annular, of the input light is maintained along the
fiber. Without the phase matching, while the initial input light
has the desired shape, this shape is not retained throughout the
traversal of the fiber. Therefore, more modal dispersion will be
present, with more light in the center of the fiber, if phase
matching is not used.
In addition to efficiently coupling the light into the fiber, the
phase matching coupler 12 also reduces the power being fedback into
the light source 10. Since the phases are matched, and the
reflected light will not have the same phase as it did when
originally incident on the phase matching coupler 12, the phase
matching coupler 12 will not return the light back to the light
source as it came. In other words, when the reflected light
traverses the system, it will be further deflected by the phase
matching coupler 12, thereby reducing the power fedback into the
light source 10.
The back reflection reduction of the phase matching coupler only
operates sufficiently when the phase matching coupler 12 is far
enough away from the fiber so that the phase is sufficiently
changed to prevent being redirected in the same manner. In other
words, if the phase matching coupler 12 is placed in contact with
the end of the fiber, while the coupler will still serve to
maintain the input distribution, since the reflected light will
have essentially the same phase as the input light, the light will
be returned substantially back to the light source as it came.
However, if the phase matching coupler 12 is placed at least
roughly three times the diameter of the beam incident on the fiber,
there is sufficient alteration of the phase due to traversal that
the reflect light incident on the phase matching coupler 12 will be
further deflected.
Further reductions to the amount of light being fedback to the
light source 10 may be realized by using a lens 30 in addition to
the phase matching coupler 12 as shown in FIG. 3. This lens 30 is
used to shape the light to provide additional reduction in the
power fedback to the light source. The lens 30 is preferably a
diffractive surface that is a combination of a lens function having
radially symmetric terms with a negative axicon function. When the
phase matching coupler 12 is spaced away from the fiber, the lens
30 may simply form a ring, since the phase matching coupler will
prevent the light from being retraced. As shown in FIG. 3, the lens
30 is on a first surface 34 of a wafer 32. The phase matching
coupler 12 is provided on a second surface 36 of the wafer 32,
opposite the first surface. The thickness of the wafer 32 controls
the numerical aperture of the image. Alternatively, the phase
matching coupler 12 may be formed on the same surface as the lens
30.
The lens 30 allows an annular intensity ring to be optimized for
the particular fiber 14. Also, by forming this ring prior to the
phase matching coupler 12, a smaller radial segment of the phase
matching coupler is used. As can be seen from equation (2), as m
increases, the amount of phase twist increases. Thus, rays at the
center of the phase matching coupler 12 receive a larger skew angle
that rays at the edge of the phase matching coupler. By shaping the
light into an annulus, this central portion is avoided, reducing
the aberrations introduced by the phase matching coupler 12. Again,
the light reflected back from the fiber 14 will not have the same
phase as the light incident on the phase matching coupler 12, so
the light will be further deflected by the phase matching coupler
12. Since the deflection angles are now altered from that of the
light source, the lens 30 will not focus the light back onto the
light source, but will further deflect the light away from the
light source.
Another embodiment is shown in FIG. 4. Here, the phase matching
coupler 12 is not used, only a reciprocal, phase sensitive system
40. An optical element will map an optical distribution, i.e.,
amplitude and phase distribution in an input plane to an output
plane. If an optical element is a reciprocal optical, it will map
the same optical distribution in an output plane back to the
original optical distribution in the input plane, as long as the
light has the same phase and intensity profile. Optical systems
that perform one-to-one mapping, such as an imaging lens, are
reciprocal, but are also phase insensitive when performing a
mapping between an object plane and an image plane, i.e., a change
in phase will not affect the mapping between the image and object
planes. However, other optical systems, such as those that perform
a one to many mapping, i.e., in which one point in the input plane
is mapped to more than one point in the output plane, while
reciprocal, are typically phase sensitive. In other words, a phase
change will alter how the light in the output plane is returned to
the input plane. An example of such a system is a negative
axicon.
In the preferred embodiment, this system 40 also creates an
intensity ring on the plane at which the fiber 14 is located. The
reflection from the fiber creates a ring back onto the system 40,
but the phase of the light has been altered due to the reflection.
This change in phase results in the light traversing the system 40
having an increased diameter of the ring in the object plane,
rather than returning the ring to the point source of the light
source. This increased diameter results in most of the light
missing the input of the light source, significantly reducing
feedback. Any other reciprocal, phase sensitive system that results
in most of the light avoiding the light source may be used. The
phase matching coupler 12 may still be employed in any position to
increase coupling bandwidth and/or enhance the feedback
suppression.
FIGS. 5A and 5B schematically illustrate different configurations
of the multi-mode coupler of the present invention. A light source
110, such as a VCSEL, an edge emitting laser or a single mode
fiber, outputs light which is incident on a beam shaper 113 which
shapes the beam and an optical element 112 together forming the
coupler. The light is then supplied to a core 116 of a multi-mode
fiber 114.
The end face of the fiber is typically located near the image plane
of the optical system as determined by the focal length of the beam
shaper 113 and the object distance, i.e., the distance from the
light source 10 to the beam shaper 113. If the fiber 114 is placed
substantially further than a depth of focus away from the image
plane, then the beam will be bigger than the core 116 of the fiber
114, resulting in less light being coupled to the fiber 114.
The optical element 112 may direct light away from a center of the
core 116 of the fiber 114 by, e.g., increasing the angle of light
in the center of the beam so that light in the center will be
incident on the outer edges of the core 116 of the fiber 114 or by
delivering no light to the center of the core. Thus, either no
light is delivered to a center 116 of the fiber 114 or any light
which is incident on the center 116 of the fiber 114 will be
incident at a high enough angle to be coupled into the desired
higher order modes.
In addition to the optical element 112, a beam shaper 113 may be
provided in either embodiment. The beam shaper 113 may be
integrated with the optical element 112 on a same surface or on an
opposite surface of the same structure. The beam shaper 113 may
also be closely spaced to the optical element 112. As shown in FIG.
5B, the beam shaper 113 may be placed at a specific distance from
the light source 110, with the optical element 112 being very close
to or even flush with the fiber 114. Each embodiment has attendant
advantages and disadvantages as discussed below.
The beam shaper 113 performs a one-to-one mapping from the input
plane to the output plane thereof. The performance of the beam
shaper may be evaluated using ray tracing. Typically, the beam
shaper 113 is used for focusing the beam output by the light source
110, which will usually be on the order of a several hundred
microns in the plane of the beam shaper, to a diameter which is
smaller than the diameter of the core, which is usually on the
order of 50 microns. If the beam shaper is a lens, theoretically,
light is focused to a point. But in reality, if the light incident
on the lens has a Gaussian profile, the light output from the lens
will still have a Gaussian profile. Another useful beam shaper for
the coupling of the present invention is a super-Gaussian element.
A super-Gaussian element converts an input beam of a particular
intensity distribution into a beam with a super-Gaussian
distribution, thereby providing a focused output beam having a
flatter peak and a much faster fall off to zero than a normal
Gaussian beam. Thus, such a beam has a fairly uniform power
distribution across the peak, pushing more power to the edges and
leaving less in the center as compared to a normal Gaussian. When
the optical element serves as an optical profile altering element,
the beam shaping and the optical element may be formed on a single
surface.
While the ratio between the distance from the light source 110 to
the beam shaper and the distance from the beam shaper to the fiber
114 shown in FIGS. 5A and 5B, in which an edge emitting laser is
used as the light source, is typically 2:1, when using a VCSEL as
the light source 110, this ratio is typically closer to 1:1. The
actual ratio will depend on the numerical aperture of the source
and the numerical aperture of the fiber. Further, depending upon
the desired coupling, the beam incident on the fiber may be smaller
than the core or larger than the core. For most applications,
source and fibers, the ratio will be between 1:4 and 4:1.
There are three primary design approaches for achieving the desired
shaping by the optical element 112. The radiation profile of light
having traversed a first embodiment of the optical element 112 is
shown in FIG. 6. As can be seen in FIG. 6, the radiation profile
has been altered by the optical element 112 to be bimodal. This
bimodal distribution is Gaussian shaped for each peak, each peak
being centered on an absolute angular value between zero and
.theta.max, where .theta.max is the critical angle for the
multi-mode fiber 114.
In the first design, the optical element 112 is a diffractive
diffuser which diffuses, i.e., substantially each point of light
incident on the diffuser substantially contributes to substantially
every point of light in the output plane, the light into the
desired angular distribution. The angles will all be less than the
critical angle for the fiber 114. Thus, if there is change in the
output profile of the light from the light source, which is of
particular concern when using a VCSEL as the light source 110, the
coupling to the multi-mode fiber will not be affected.
Additionally, if the diffractive diffuser does not also provide
collimation or focusing to the light, precise alignment of the
diffractive diffuser is not needed.
A diffractive diffuser may be formed by setting the fast Fourier
transform (FFT) to be a ring, i.e., set the fringe period of the
diffractive between the two values bounding the ring. In order for
the diffractive diffuser serving as the optical element 112 to
function properly, it must be positioned at least more than a width
of the core, preferably at least three to five times the width of
the core, away from the fiber. This placement ensures that
substantially every point of light incident on the diffuser
substantially contributes to substantially all of the pattern
incident on the fiber. Such a Fourier transform diffractive
diffuser may be realized in accordance with U.S. Pat. No.
5,850,300, which is hereby incorporated by reference in its
entirety.
The diffractive diffuser preferably alters the angular distribution
of the light into any desired angular distribution which will
efficiently coupler the light into the higher order modes of the
multi-mode fiber. This desired angular distribution will typically
be a ring, an annulus or a grid of N spots, but may be any other
desired angular distribution for a particular multi-mode fiber. For
example, a radial grating may be provided which sends a significant
portion of the light, e.g., 80%, into the .+-.1 order and randomly
varies the period to provide the range of desired angles radially
away from the center. Further, a ring or a multipole of N spots,
e.g. a quadropole of 4 spots, where N is an integer greater than or
equal to one, may be realized by providing a grating to create
spots located at r.sub.N, where r.sub.N is a distance from the
center to the spot. Additionally, the diffractive diffuser may be a
binary element which splits the light into two beams directed to
the periphery of the fiber core.
While a Fourier transform diffractive diffuser as described above
is useful when employing a light source having an unstable output
beam profile, this diffractive diffuser is difficult to use in the
configuration of FIG. 5B, since the optical element 112 is too
close to the fiber 114 for a Fourier transform diffractive diffuser
to create, for example, a ring on the end face of the fiber. If the
optical element 112 is more than a few wavelengths away from the
end of the fiber, a diffuser serving as the optical element 112
will function properly. Additionally, diffusers often have lower
efficiencies than other optical elements. The following two designs
may be used with either configuration of FIGS. 5A and 5B.
A generic embodiment of the second design is illustrated in FIG. 7.
As can be seen therein, the optical element 112 is composed of a
central region 118 and a peripheral region 120. The central region
118 and the peripheral region 120 affect the beam incident thereon
differently. These different regions may be discretely different,
include subregions of different functioning, and/or may
continuously vary the treatment of the light from the center to the
periphery. For example, the central region 118 deflects the light
incident thereon away from the center. The peripheral region 120
may not affect the light incident thereon at all, or it may be
designed to, for example, collimate the light incident thereon.
Using such an element allows the light in the center of the beam
which would have been incident on the center of the fiber to be
deflected away to edges of the fiber, while not imposing an
increase in the angle on the light near the edge of the beam which
would already be incident upon the desired portion of the fiber.
Alternatively, although not as efficiently, the central region may
simply block the light incident thereon to form the desired ring
shape.
A specific embodiment of the second design is illustrated in FIG.
8A. The optical element 112 provides a one to one mapping of each
point to the fiber, while continuously varying the element
encountered by the light from the center to the periphery. A
converging portion 150 of the coupler 112 converges, i.e., reduces
the incident angle, of light at the outer edge of the beam. A
diverging prism 152 of the optical element 112 diverges, i.e.,
increases the angle of the light, of light in the center of the
beam to prevent light from hitting the center of the fiber. Another
specific embodiment of one to one mapping is shown in FIG. 8B in
which a diverging portion 154 is located in the center of the
optical element again to diverge light in the center of the beam.
FIGS. 8A and 8B are radially symmetric.
Another specific embodiment of the second design is shown in FIG.
9. FIG. 9 is a cross-section of a prism. If this cross-section is
used to form a linear prism, such that there is a variation in
thickness along the axis coming out of the plane of the page, two
spots will be generated in the image plane of the system. When a
linear prism is combined with a lens function, the cross-section
will look like FIG. 8B, but will not be radially symmetric, since
the linear prism is not radially symmetric. If the cross-section in
FIG. 9 is rotated radially to form a radial prism, a ring will be
generated in the image plane of the system. If the radial prism is
combined with a lens function, the cross-section will look like
FIG. 8B and will be radially symmetric.
In the embodiments of the second design, light near the edge of the
beam can be mapped to the edge of the fiber with little or no
increase in the angle. Light from the center of the beam can be
mapped to the edges of the fiber. Where the optical element 112 is
illustrated as a refractive element in the embodiments of the
second design, the optical element 112 may be designed as a
diffractive element using the known diffractive approximation of
the refractive element, either as a continuous diffractive or as a
discrete diffractive. Preferably, the diffractive elements are
computer generated holograms.
The same effect as provided by configurations of the second design
may be realized by providing an optical element having diffuser
patches having finer features and/or smaller periods closer to the
center and larger features and/or larger periods towards the
periphery or nothing at the periphery. At the edge of the element
the light is not affected, or has a small increase in angle, and
the light at the center is diffused to increase the angle of light
towards the center. As long as the diffuser patches are distributed
on the optical element so that it does not treat the center and the
periphery in the same manner, e.g., a diffuser only at the center
or a gradient diffuser, the diffuser patches may be used next to
the end face of the fiber, such as shown in FIG. 5B. Such diffusing
patches may also be multiplexed with any desired lens function.
Further, while the embodiments of the second design have been
discussed with reference to the optical element 112, the second
design may also be used as the beam shaper with the optical element
of the first design or the optical element of the third design,
discussed above. Further, when using a diffractive diffuser which
splits the light into two beams directed to the periphery of the
fiber core, this element does not have to be unitary, but may be
split into half. In such a configuration, the two elements serve as
a beam shaper, with one half mapping the light incident thereon to
one point and the other half mapping the light incident thereon to
another point.
The above discussion has assumed that the ideal radiation pattern
for coupling light into the fiber is a ring. Generally, the ideal
radiation pattern, and hence the desired angular distribution, will
be a function of the properties of the fiber, i.e., where
propagation is most efficient. The design of the coupler for
achieving the desired angular distribution will also depend on the
radiation profile output by the light source used to illuminate the
fiber.
Any of the above designs may be integrated with other optical
functions, such as collimation, in a single element, as shown in
FIGS. 8A, 8B and 9. For the integration of the coupler with
additional optical functioning, the additional functioning may be
multiplexed with the shaping function, as disclosed in commonly
assigned, co-pending application U.S. patent application Ser. No.
09/296,397 filed Apr. 23, 1999, entitled "Diffusing Imager and
Associated Methods" which is hereby incorporated by reference in
its entirety. Further, any of the above designs may be integrated
with the other elements of the light source/fiber system, including
further optical elements as discussed above.
While the present invention is described herein with reference to
illustrative embodiments for particular applications, it should be
understood that the present invention is not limited thereto. Those
having ordinary skill in the art and access to the teachings
provided herein will recognize additional modifications,
applications, and embodiments within the scope thereof and
additional fields in which the invention would be of significant
utility without undue experimentation.
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